The main purpose of the burner is to mix fuel and air together to obtain stable and efficient combustion with good flame stability and the smallest possible amount of NOx emissions. Therefore, the burner design must ensure that the proper amounts of fuel and air are introduced in the right locations within the burner and that these amounts of fuel and air are thoroughly mixed, so that complete combustion takes place with a minimum amount of excess air in order to achieve optimum overall efficiency.
The two burner principles, which could be combined to use their respective advantages, are the premix combustion burner and the diffusion flame burner.
In the premix combustion burner, the air, required for combustion, is mixed with the burner fuel before delivery to the combustion zone. The better the mixing of fuel and air the less hot zones with a fuel/air ratio exceeding the stoichiometric requirements exist. Since flame temperature is the dominant factor driving NOx production it follows that the more fuel lean the mixture, the lower the NOx produced.
In the diffusion flame burner, the fuel is not mixed with the air ahead of the combustion zone, but delivered as pure fuel in the immediate vicinity of the combustion zone. Diffusion flame burners provide good flame stability. The NOx production is relatively high.
Low emission gas turbine engines often use a combustor with two operating modes including a pilot nozzle that forms a diffusion flame and a plurality of main nozzles for discharging a fuel/air mixture to form premixed flames as the main combustion around the diffusion flame. The U.S. Pat. No. 5,901,555 describes a conventional gas turbine with the main burners divided into a plurality of groups in accordance with the load. The flow rate of the pilot fuel is increased when the gas turbine load is low, to achieve stable combustion. When the gas turbine load is high, the ratio of the pilot fuel is decreased, to decrease the amount of NOx. Separately controllable fuel lines, valves, pipe work and a control logic are required to achieve the appropriate fuel flows to the pilot and main nozzles, increasing the cost of the engine.